Poetry that means a lot to the author but little or nothing to
anyone else. I think this is partly because many people write poetry
as a form of therapy when going through the rough parts of life, and
the stuff one writes while depressed tend to be inchoherent,
self-pityingand/orintrospective. There's nothing
wrong with writing that kind of stuff if it makes you feel better, but
you should really think twice before letting anyone else read it.

Poetry that only says things that've been said before (and
better). There are some themes you should try to stay clear of if you
want to be taken seriously (like 'eternal love', 'fading roses', and
almost anything to do with vampires), escpecially if haven't
got anything original to offer. Okay, there's nothing inherently
wrong with imitating another poet as a learning experience, but
again, you should think twice before showing it to anyone else.

Poetry written by someone not at ease with the language. By this I
mean poetry with contrived or awkward rhymes, or written in a
meter the author couldn't handle. Free verse is far more difficult
than it seems at first, and not everyone is a Whitman
or an E.E. Cummings. This is mostly a matter of experience,
though. The more you write, the better you get and you acquire your
own voice and style.

There are exceptions to every rule; expecially those
listed above. Some really great pieces have been the fruits of angst
and depression, some really, really good ones are nonsensical
(take 'Jabberwocky' for instance), and there are some poets who can
blow life into even the most worn-out themes.